Ultrasound Imaging and Therapeutics Research Laboratory

Optical Elastography for Assessment of Myocardial Regeneration

Andrei Karpiouk , Kirill Larin, Ph.D

Summary

Optical coherent elastography (OSE) is an emerging imaging technique based on optical coherent tomography (OCT) principles which also can deduce some mechanical properties of the imaged tissues.1 An ability of phase-sensitive OCT (ps-OCT) to detect extremely small, less than 100 nm, displacements of tissues enables detection of an elastic wave propagation that travels inside soft tissue in response to external excitation. A shear wave can be intentionally generated in the soft tissues mechanically [2], by ultrasound [3], laser pulse [4], air puff [5], Lorentz force [6], etc. Given the density distribution of the soft tissues, those shear elastic modulus can be calculated.7

This project employs OSE with ultrasound excitation of shear waves to assess the stiffness of heart muscles. Myocardial infarction results in damage of cardiac muscle which becomes stiffer rather than benign muscle tissues. Currently, several therapeutic approaches to enable effective mechanisms of myocardial regeneration are under development, but there is still a lack of methods to map the shear elasticity of cardiac muscle and assess efficiency of the developed approaches. Since the developed methods were tested using animal models, there is still a need in the compact OSE probe to assess the stiffness of heart muscle with minimal surgical impact. To avoid motion artifacts caused by heart beating, the probe should be capable of do all measurements fast, between heart beats, while the heart is effectively stationary.8

Figure 1 shows a diagram and microphotograph of combined OSE probe, experimental setup and detected shear waves in the heart muscle ex vivo. Two ultrasound transducers and OCT probe were integrated into a single device. The probe was positioned above the rabbit’s heart. Two shear waves were generated by each ultrasound transducer separately, one after another, while ps-OCT system detected the propagation of the waves in the heart muscle. Given the known distance between the transducers, the speed of swear wave and, therefore, shear elasticity of the heart muscle were calculated. Future work includes the development of more compact and effective ultrasound transducers, dual-channel power amplifier, and software to enable mapping of heart muscle elasticity in real time. At the same time, this technology should be tested in vivo using healthy and diseased animals.

OCT
Figure 1. (A) Schematic design and micro-photograph of the two-transducer OSE probe. Bar is 2 mm. (B) Design of the experiment. Bar is 2 mm. (C) Displacements of heart muscle caused by shear wave propagating through the detection point. The noticeable shift of one of the shear waves is explained by additional traveling distance and allows to calculate its speed.

References

  1. A. P. Sarvazyan et al., “Shear wave elasticity imaging: a new ultrasonic technology of medical diagnostics,” Ultrasound Med. Biol. 24, 1419–1435 (1998).^
  2. C. Li, G. Guan, R. Reif, Z. Huang, and R. K. Wang, “Determining elastic properties of skin by measuring surface waves from an impulse mechanical stimulus using phase-sensitive optical coherence tomography,” J. R. Soc. Interface 9(70), 831–841 (2012).^
  3. O. A. Ejofodomi, V. Zderic, and J. M. Zara, “Development of novel imaging probe for optical/acoustic radiation imaging (OARI),” Med. Phys. 40(11), 111910 (2013).^
  4. C. Li, G. Guan, Z. Huang, M. Johnstone, and R. K. Wang, “Noncontact all-optical measurement of corneal elasticity,” Opt. Lett. 37(10), 1625–1627 (2012).^
  5. S. Wang, A. L. Lopez 3rd, Y. Morikawa, G. Tao, J. Li, I. V. Larina, J. F. Martin, and K. V. Larin, “Noncontact quantitative biomechanical characterization of cardiac muscle using shear wave imaging optical coherence tomography,” Biomed. Opt. Express 5(7), 1980–1992 (2014).^
  6. C. Wu, M. Singh, Z. L. Han, R. Raghunathan, C. H. Liu, J. S. Li, A. Schill, and K. V. Larin, “Lorentz force megahertz optical coherence elastography,” J. Biomed. Opt. 21(9), 090502 (2016).^
  7. J. F. Doyle, Wave Propagation in Structures (Springer-Verlag, New York, 1997).^
  8. Andrei B. Karpiouk, Donald J. VanderLaan, Kirill V. Larin, Stanislav Y. Emelianov, “Integrated optical coherence tomography and multielement ultrasound transducer probe for shear wave elasticity imaging of moving tissues,” J. Biomed. Opt. 23(10), 105006 (2018).^